Direct broadcast systems are now in use for sending video and audio signals to subscribers. In such systems, a transmitter provides analog signals representing video images and digital signals representing audio information. The analog and digital signals from the transmitter are sent to a satellite which then relays the signals to subscribers at different locations. The receiver at a subscriber location then converts the analog signals to an image represented by such signals or converts the digital information to sounds represented by such signals. There are now about ten million (10,000,000) subscribers to digital broadcast systems in Europe.
In direct broadcast systems, the frequencies are divided into bands, each band having a range of approximately ten Megahertz (10 MHz). In each band, approximately six Megahertz (6 MHz) are used for the analog video signals for a video channel and the remaining four Megahertz (4 MHz) are used to provide digital audio signals for up to twenty four (24) audio channels, each having an individual and a limited range of frequencies within the frequency band. Each audio signal consists of a compressed digital bit stream incorporating forward error correction which is modulated using Quadrature Phase Shift Keying (QPSK) onto a subcarrier in the 6-10 MHz frequency band of the satellite transponder.
It is sometimes difficult at times to select a particular one of the audio channels in the frequency band. This results partly from the fact that the digital audio signals in the particular channel have a limited range of frequencies and have a relatively low power level, particularly in relation to the power level in the analog video signals in the same frequency band. This has caused noise to be mixed with the sound recovered from the digital audio signals. Because of this, the quality of the sound recovered from the audio signals in the particular channel is sometimes low. These problems have been known to exist for some time. A considerable effort has been made, and significant amounts of money have been expended in this effort, to resolve these problems. However, the problems continue to exist.